1. Field of the Invention
The present invention relates to an image pickup signal processing apparatus for A/D-converting an output signal from a solid-state image pickup element and for extracting a luminance signal and color signals.
2. Related Background Art
FIG. 1 is a schematic view of an interline CCD used as a conventional solid-state image pickup element. An interline CCD 1 comprises a plurality of photodiodes 2 for converting light incident on a light-receiving unit of the CCD 1 into charges and accumulating the charges, and a vertical transfer unit 3 for transferring the charges from the photodiodes 2 in units of stages per 1H (one horizontal period). The vertical transfer unit 3 includes transfer electrodes V1 to V4. The transfer electrode V1 also serves as a transfer gate for transferring the charges of the odd-numbered lines of the photodiodes 2 to the vertical transfer unit 3. The transfer electrode V3 serves as a transfer gate for transferring the charges of the even-numbered lines of the photodiodes 2. The vertical transfer unit 3 is driven in response to 4-phase transfer pulses. The interline CCD also includes a horizontal transfer unit 4 for horizontally transferring the charges transferred in units of stages each per 1H, an output amplifier 5 for converting the charge into a voltage and outputting the voltage, and an output terminal VOUT. The horizontal scan unit 4 includes transfer electrodes H1 and H2.
FIG. 2 shows the layout of a color filter arranged on the CCD 1. This filter comprises a stripe filter in which three different colors are repeated in an order of Ye (yellow), G (green), and Cy (cyan) in the horizontal direction.
FIG. 3 is a timing chart for reading the charges from the CCD 1. For example, at time t1 of the A field, the transfer electrodes V1 and V3 are set at Hi (high) level, so that charges of the two adjacent lines of the photodiodes 2 are added to each other. The sum is transferred through the vertical transfer unit 3 in units of stages each per 1H toward the horizontal scan unit 4. The charge transferred to the horizontal scan unit 4 during a horizontal blanking period is output from the output amplifier 5 in an order of Ye, G, Cy, . . . .
FIG. 4 is a block diagram showing a circuit arrangement of a conventional image pickup signal processing apparatus for processing an output signal from the CCD 1. The image pickup processing apparatus includes a CDS circuit 6 for eliminating noise of the CCD 1, a clamp circuit 7 for fixing the output potential of the CDS circuit 6 at a predetermined value, an A/D conversion circuit 8 for performing A/D conversion, a frame memory 9, a digital signal processing portion 10 for performing arithmetic operations of the data obtained from the frame memory 9 through a data bus 11 and for generating a luminance signal and color signals, a D/A conversion circuit 12 for D/A-converting a luminance signal output from the digital signal processing portion 10, D/A conversion circuits 13 and 14 for D/A-converting color difference signals, respectively, a low-pass filter (LPF) 15 for limiting the band of the luminance signal, and low-pass filters 16 and 17 for limiting the bands of the color difference signals. The low-pass filters 15, 16, and 17 output a luminance signal Y and color difference signals R-Y and B-Y, respectively.
FIG. 5 is a block diagram showing a detailed arrangement of the digital signal processing portion 10. The processing portion 10 includes a latch circuit 18 for reading out the Ye signal from the frame memory 9 and latching the readout Ye signal, a latch circuit 19 for latching the G signal as in the Ye signal, a latch circuit 20 for latching the Cy signal as in the Ye signal, a subtracter 21 for subtracting the G signal from the Ye signal to generate the R (red) signal, a subtracter 22 for subtracting the G signal from the Cy signal to generate the B (blue) signal, a gamma correction circuit 23 for gamma-correcting the signals read out from the frame memory 9 in an order of Ye, G, Cy, . . . and outputting a digital luminance signal Y(D), a gamma correction circuit 24 for gamma-correcting the readout data into the R signal, a gamma correction circuit 25 for gamma-correcting the readout data into the G signal, a gamma correction circuit 26 for gamma-correcting the readout signal into the B signal, and a matrix operation circuit 27 for performing matrix calculations of the gamma-corrected R, G, and B signals to generate digital color difference signals R-Y(D) and B-Y(D).
In the image pickup processing apparatus having the above arrangement, differences between complementary color signals output from the CCD 1 and digitized by the A/D conversion circuit 8 are calculated by the subtracters 21 and 22 in the digital signal processing portion 10 to generate primary color signals. All the signals are gamma-corrected and then converted into analog signals again. The converted analog signals are then output to an external circuit.
In the conventional image pickup processing apparatus, the differences between the complementary color signals from the solid-state image pickup element and converted into digital signals by the A/D converter are calculated to generate the primary color signals. In a quantization process using about 8 bits, the bit resolution of the color signals upon completion of the calculations of the differences is degraded. The influence of quantization noise cannot be neglected, and an S/N ratio of the color signal is undesirably decreased.